1. Field of the Invention
This invention is directed to methods for enhancing the adhesion of composite electrodes onto metal foils (i.e., current collectors), to metal foils prepared by these methods and to electrochemical cells produced from these metal foils.
2. State of the Art
Electrochemical cells comprise a cathode, an anode and an electrolyte interposed therebetween. The electrochemical cells are often defined as liquid or solid cells and this refers merely to whether the electrolyte interposed between the anode and the cathode is a liquid or a solid. Solid electrochemical cells are well known in the art and present many advantages over conventional liquid batteries such as improved safety features, lighter weight, etc.
In order to enhance the overall current produced by solid or liquid batteries, it is conventional to employ several electrochemical cells in a battery. When so employed, the current from each of the cells is accumulated so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrochemical cells employed in the battery.
One method for accumulating the current from individual electrochemical cells is by using a current collector attached to the cathode or the anode of the electrochemical cell. Typically, the current collector is a metal foil which is coupled to other current collectors in the battery so that the current generated by each cell is collected and accumulated over all of the cells. Thus, the total current generated by the battery is a summation of the current generated by each of the electrochemical cells employed in the battery minus whatever current is lost due to resistance in the current collector.
Notwithstanding the benefits of using current collectors (i.e., metal foils) in electrochemical cells, there is a problem when current collectors are used in conjunction with composite electrodes (i.e., electrodes derived from a composite of different materials). Specifically, composite electrodes are typically prepared from a paste which is applied onto the surface of the current collector and then cured to form the electrode.
For example, one conventional method for forming a composite cathode is by first forming a cathode paste comprising a compatible cathodic material (e.g., LiV.sub.3 O.sub.8, V.sub.6 O.sub.13 and the like), a conductive material such as carbon powder, an electrolytic solvent, and a prepolymer or a mixture of prepolymers. The cathode paste is then coated at a set thickness by extrusion or other suitable application means onto a metal foil which acts as a current collector and the paste is then cured to provide for the solid cathode.
However, the metal foils used as current collectors (e.g., aluminum foil, nickel foil, etc.) typically lack adequate adherence so as to permit a paste to adequately adhere to the foil. Thus, during application of a paste onto the foil, the paste can become dislodged from the foil. In turn, after curing, such dislodgement will result in defects in collecting current from that cell.
One method disclosed in European Patent Application Publication No. 0 397 523 ("EPA '523") for overcoming the inadequate adherence of metal foils to an electrode paste is to roughen the surface of the metal foil with electrolytically deposited metal. The electrolytically deposited metal "roughens" the surface of the foil and one commercially employed current collector is a roughened nickel on nickel current collector which is available as CF18/NiT from Fukuda Metal Foil & Powder Company, Ltd., Kyoto, Japan. The electrolytic deposition process results in the formation of a roughened surface characterized on a microscopic level by peaks and valleys rather than a smooth planar surface (see FIGS. 3, 4 and 6 of EPA '523). Such roughened surfaces lead to a more adherent surface.
On the other hand, roughened metal on metal foils are rather expensive and result in undesirable weight increases for the metal foil current collector which translates into an undesirable increased weight for electrochemical cell itself. Moreover, electrochemical deposition of metal is not practical for certain metal foils. For example, aluminum foil typically contains an outer passivation layer of aluminum oxide which inhibits the electrochemical deposit of aluminum onto the surface of the aluminum foil.
On the other hand, the art discloses that polymers containing grafted acid functionalities (i.e., maleic acid or anhydride) have improved adherence to metal foils, such as aluminum foil, used in food wraps. See U.S. Pat. Nos. 4,810,755 and 4,857,600 as well as International Patent Application Publication No. WO 91/02760. U.S. Pat. No. 4,810,755 further discloses the inclusion of a coloring agent such as carbon black into the polymer. However, neither U.S. Pat. No. 4,810,755 nor the other references disclose the inclusion of a effective amount of a conductive material in the polymer so as to render the polymer conductive or that the use of such amounts of conductive material would still result in the enhanced adherence of the polymer.